1. Field of the Invention
The present invention pertains to systems for indicating measured values and, more particularly, to systems for indicating the speed of movement of a motor vehicle.
2. Discussion of the Prior Art
Eddy current measuring mechanisms are conventionally utilized to indicate the speed of movement of a motor vehicle, such eddy current measuring mechanisms being formed, essentially, of a magnet rotatably driven by a flexible shaft such that the magnet, as a result of the formation of eddy currents, rotates a bell of conductive material coupled with a pointer to move the pointer at an angle dependent on the rate of revolution of the magnet. Tachometers of this type have many disadvantages including accuracy limited by the tolerances of the mechanical structure, the properties of the magnet, the electrical conductivity of the bell and manual calibration, undesirable size and space requirements, these flexible shafts used for driving the magnets have a limited life and tend to cause undesirable noises, and the rate of revolutions per distance indicative of the transmission ratio between distance covered and the number of revolutions of the flexible shaft must be mechanically coupled into the tachometer and is, therefore, relatively expensive.
Another mechanism for indicating the speed of movement of a motor vehicle utilizes a moving coil measuring system cooperating with a transmitter having a pulse forming stage to produce a pulse frequency corresponding to speed. Such systems utilize a frequency voltage transformer to produce an analog value which is supplied to an additional adapter stage, the voltage transformer converting the rate of revolutions per distance to an analog value for use in the system. The accuracy of such systems, especially over long periods of time, is limited not only by their dependence on analog values, but, further, due to their dependence upon the characteristics of the magnet of the moving coil measuring system, and such systems further suffer from the disadvantages of requiring the moving coil measuring system to be sufficiently damped to permit quiet operation and of being of an undesirably large size.
It is known to drive a kilometer counter by means of a stepper motor rotatable only in a single direction, the stepper motor receiving pulses generated by a transmitter via a transducer, the transmitter including two analog integration stages for converting the rate of revolutions to distance. A monostable multivibrator is coupled with the output of each integration stage; and, two integration branches, each formed of an integration stage and a multistable multivibrator, are connected in such a way to alternately control each other. The monostable multivibrators generate defined output pulses of constant duration which are fed to the field coils of the stepper motor. Such systems have limited accuracy, however, due to their operation primarily in an analog mode with inaccuracies being especially prevalent in the integration stages which require pulses supplied thereto of a precisely defined duration. Another disadvantage of such systems is the expense thereof.